Prostate cancer is the second leading cause of death from cancer among men. In fact, prostate cancer is the most common non-cutaneous cancer diagnosed in the American male. The number of men diagnosed with prostate cancer is steadily increasing as a result of the increasing population of older men as well as a greater awareness of the disease leading to its earlier diagnosis (Parker et al., 1997, CA Cancer J. for Clin. 47:5-28). It was projected that over 334,500 men would be diagnosed with prostate cancer in 1997, and that approximately 41,800 deaths would result from the disease. The life time risk for men developing prostate cancer is about 1 in 5 for Caucasians, and 1 in 6 for African Americans. High risk groups are represented by those with a positive family history of prostate cancer or African Americans. Over a lifetime, more than ⅔ of the men diagnosed with prostate cancer die of the disease (Wingo et al., 1996, CA Cancer J. for Clin. 46:113-25). Moreover, many patients who do not succumb to prostate cancer require continuous treatment to ameliorate symptoms such as pain, bleeding and urinary obstruction. Thus, prostate cancer also represents a major cause of suffering and increased health care expenditures (Catalona, 1994, New Eng. J. Med. 331:996-1004).
PSMA is a 120 kDa molecular weight protein expressed in prostate tissues and was originally identified by reactivity with a monoclonal antibody designated 7E11-C5 (Horoszewicz et al., 1987, Anticancer Res. 7:927-935; U.S. Pat. No. 5,162,504). PSMA was obtained in purified form (Wright et al., 1990, Antibody Immunoconjugates and Radio Pharmaceuticals 3:Abstract 193) and characterized as a type II transmembrane protein having sequence identity with the transferring receptor (Israeli et al. 1994, Cancer Res. 54:1807-1811) and with NAALADase activity (Carter et al., 1996, Proc. Natl. Acad. Sci. U.S.A. 93:749-753). More importantly, PSMA is expressed in increased amounts in prostate cancer, and elevated levels of PSMA are also detectable in the sera of these patients (Horoszewicz et al., 1987, supra; Rochon et al., 1994, Prostate 25:219-223; Murphy et al., 1995, Prostate 26:164-168; and Murphy et al., 1995, Anticancer Res. 15:1.473-1479). A cDNA encoding PSMA has been cloned (Israeli et al., 1993, Cancer Res. 53:227-230), and it produces two alternatively spliced mRNA species: an mRNA species containing 2,653 nucleotides that encodes PSMA, and a second mRNA species containing 2,387 nucleotides referred to as PSM′ (Su et al., 1995, Cancer Res. 55:1441-1443). Prior to the present invention, it was not known whether PSM′ encoded a protein product or existed only as an untranslated mRNA species because a PSM′ protein product had never been detected. A recent report by Carter et al. (1996, Proc. Natl. Acad. Sci. U.S.A., 93:749-753) shows a high degree of identity between 1428 bases representing a portion of the PSMA cDNA and the cDNA sequence of protein N-acetylated α-linked acidic dipeptidase (NAALADase). NAALADase has enzymatic activity towards the neuropeptide N-acetylaspartyl glutamate to yield glutamate and N-acetylaspartate. This report demonstrates NAALADase activity inherent to PSMA protein, but the catalytic portion of PSMA was not identified. NAALADase activity was found in LNCaP cells which expressed PSMA, but not in PC3 cells which do not express PSMA. Transfection of the PSMA cDNA into PC3 cells produced NAALADase activity and the presence of PSMA in these cells.
The difference between the cDNA of PSMA and PSM′ is the loss of the transmembrane and intracellular coding regions containing nucleotides #1-171 of SEQ ID NO:2 or amino acids #1-57 of SEQ ID NO: 2. PSMA is described as a type II membrane protein and it is known that the functional catalytic domain of type II membrane proteins resides in the C-terminal extracellular region of the molecule (DeVries, et al., 1995, J. Biol. Chem., 270:8712-8722).
PSM′ mRNA is found in greater quantities in normal prostate tissues as compared with prostate tissues of patients with benign hyperplasia or prostate cancer (Su et al., 1995, supra). In contrast, PSMA mRNA is found in greater levels in patients with prostate cancer as compared to patients without prostate cancer (Su et al., 1995, supra). This observed difference is consistent with serum protein levels of PSMA described previously (Horoszewicz et al., 1987, supra; Rochon et al., 1994, supra; Murphy et al., 1995, supra; and Murphy et al., 1995, supra). In this connection, an elevated level of PSMA in sera of prostate cancer patients has been correlated with disease progression versus remission, and may be used as a prognostic marker (Murphy et al., 1995, supra).
The epitope recognized by monoclonal antibody 7E11-C5 has been mapped to the first 6 amino acids of the intracellular N-terminal region of PSMA (Troyer et al., 1995, Urol. Oncol. 1:29-37) (FIG. 1). Electron immunocytochemistry using 7E11-C5 has localized its epitope to the cytoplasm, and specifically to the inner leaf of the plasma membrane (Troyer et al., 1994, Proc. Am. Assoc. Cancer Res. 35:283, Abstract 1688). Furthermore, in in vitro tests, monoclonal antibody 7E11-C5 stains only fixed and permeabilized cells (Horoszewicz et al., 1987, supra), which is in accord with the mapping of the 7E11-C5 epitope to the N-terminus or intracellular domain of PSMA. While 7E11-C5 is useful for detecting prostate cancer in vivo which presumably exposes its epitope through necrosis and/or apoptosis, a monoclonal antibody specific for the extracellular domain of PSMA would allow more efficient detection of PSMA on the cancer cell surface. In addition, monoclonal antibody 7E11-C5 does not recognize PSM′, since PSM′ lacks the intracellular domain of PSMA, based on the sequence of its mRNA transcript.
Citation or identification of any reference in this section or in any other section of this application shall not be construed as an admission that such reference is available as prior art to the present invention.